The present invention relates to a motor drive control which is applicable to, for instance, an electronic throttle control apparatus for vehicles for driving a throttle valve disposed in an engine intake pipe by a direct current motor in accordance with a pedal stroke of an accelerator pedal and the like.
U.S. Pat. No. 5,712,550 (Japanese PCT Laid-open Publication JP-A-9-501817) teaches, as shown in FIG. 16, an electronic throttle control apparatus which drives a motor M by an H-bridge drive circuit using four switching transistors Q1-Q4 and executes a current limitation while ensuring responsiveness of a valve to be driven. In this apparatus, the motor is supplied with a current of 100% duty at the time of motor drive initiation to speed up the valve operation, and is supplied with a current in a reverse direction at the time of approaching to a target valve position to brake the valve operation.
The currents for the motor drive initiation and for braking are subjected to current limitations. With this current limitation, output loss required to drive devices Q1-Q4 such as power MOSFETs or the like in the H-bridge circuit is reduced. It becomes possible to construct the drive circuit with drive devices of small rating and in low cost. It is to be noted that FIG. 16 shows a case in which a drive logic circuit 100 receives drive command signals A1-A4 from a control circuit 101 and executes a current limitation processing in response to a current detection signal from a current detection circuit 102.
On the other hand, a certain limited current is continued to be supplied for a fixed time period even under a motor lock condition, so that the operation restores from a light lock condition where the valve cannot be controlled to a target position and stays at substantially the same position. This light lock condition may occur from freezing (icing) of water remaining on the valve.
Specifically, as shown in FIG. 17, the limit of the current supplied to the motor is switched to a low value Ilimit(L) (&lt;Ilimit(H)), when a motor lock occurs during a period of limiting a motor current IM to the current limitation value Ilimit(H) at the time of motor drive initiation or braking. The motor lock is detected when the current limitation continues for a predetermined time period. The current is continued to be supplied, when the motor current IM enters a normal current zone Znor within the predetermined time period T2 as shown by (I) in FIG. 17, that is, when the lock condition disappears and normal operation restores.
The motor current supply is interrupted, when the motor current IM does not enter the normal current zone Znor after the predetermined time period T2 as shown by (II) in FIG. 17. The current limitation may be executed by detecting the motor current IM and interrupting the current supply temporarily when the detected current IM reaches the current limitation value.
Thus, the drive device can be sized small and cost can be reduced by reducing the current under the lock condition.
In the above apparatus, however, the restoration from the current limitation condition under the lock condition is determined from the restoration of the motor current IM to the normal current zone Znor, that is, when the current decreases to less than a maximum value InorMAX of the normal current zone. Therefore, the current limitation value Ilimit (L) under the lock condition need be set to a larger value relative to the normal current zone Znor, and hence the drive devices are required to be sized large to meet this current limitation condition.
More specifically, in setting the current limitation value Ilimit(L) under the lock condition, it must be taken into consideration that the current limitation operation is executed properly relative to the current normal condition of a feedback control for a target opening angle including a holding of the valve opening angle but excluding the current supply for the motor drive initiation or braking. Tolerances in manufacture of the current detection circuit and motor coils must be considered. For this reason, the current limitation value Ilimit(L) must be set to a sufficiently large value.
It is thus necessary to set the current limitation value Ilimit(L) to a larger value for the current limitation operation, when the release from the lock condition is determined from a change in the motor current (entering into the normal current zone Znor). That is, the current value does not necessarily decrease below the limitation value Ilimit(L) even when released from the lock condition, if the current limitation value Ilimit(L) is set to the maximum value InorMAX of the normal current zone Znor. Therefore, it is impossible to determine the release from the lock condition based on the change in the current value.
The drive device is required to withstand the lock-time current for the fixed time period. The output loss of the drive device increases in comparison with the case of drive current supply under the normal condition, if the value Ilimit(L) is set to be sufficiently larger than the value InorMAX. As a result, the drive device must be an expensive component which has a large rating or has a large chip size with a small on-resistance. Further, the motor must be designed on an assumption that a large torque will be applied in excess of the normal drive torque, resulting in a cost increase.